This invention generally relates to techniques for measuring properties of structural materials. In particular, the invention relates to techniques for measuring properties of curved structural materials, such as curved core material.
Sandwich panels are widely used in the aerospace industry and other industries where strong, lightweight structures are required. Such a sandwich panel typically includes a lightweight central core structure that is sandwiched between two composite or metal face sheets. The face sheets are adhesively bonded to opposite sides of the core. Reinforcing cores are often necessary in large panels to provide sufficient rigidity and structural strength in the panel so that the panel can withstand substantial loading.
Various core structures are presently in use, with the two main types being rigid foam and honeycomb. For structures having rigid foam cores, the face sheets contact the foam over a relatively large surface area, which ensures a strong bond. For structures having honeycomb cores, the surface area which is available for bonding to the face sheets is much smaller than the available surface area for foam cores. The honeycomb cells extend transversely between the face sheets so that the only surfaces available for bonding to the face sheets are the outer edges of the cells.
A typical sandwich panel has a bottom composite or metallic layer manufactured from a fibrous material that is impregnated with a resin. A core material of amorphous foam or honeycomb structure is then positioned on the bottom layer. A top composite or metallic layer is then bonded to the core. An edge layer is then applied to connect the top and bottom layers and to seal the core therebetween. This assembly is then cured in an autoclave or an oven with a vacuum bag at elevated temperatures and pressures.
The reinforcing core is typically designed to have substantial compression resistance along a core axis. The core is typically oriented with respect to the top and bottom layers so that the core axis is perpendicular thereto. In this way, the structural panel undergoes loading in a fashion similar to a beam wherein the top and bottom layers correspond to the flange portions of a beam and the core corresponds to the web portion of a beam. The core itself has little resistance to compression in a direction perpendicular to the core axis.
Various techniques are known for forming curved structural panels of the foregoing type. The curved core material can be subjected to substantial shearing forces when the curved structural panel undergoes substantial loading. Accordingly, the curved core material must be designed to have sufficient shear strength to withstand the shearing forces produced by such loading. The need to validate the shear strength of curved core material is of increasing importance due to its increasing use in airframe structures. Some existing testing methodologies use flat core material and analytical techniques to determine allowable shear strength properties for structure containing curved core material.
There is a need for an effective mechanical test method for directly measuring the shear strength of curved core materials in order to validate analytical techniques or develop design allowables. Such a test method would facilitate improved structural design and could also be used as a screening method to evaluate potential curved core materials in the proposed use state.